The couple -- who first debuted their relationship in May 2008 at the Cannes Film Festival -- got into a fight over the weekend at a party Samantha threw for her fashion-designer sister Charlotte Ronson at West Hollywood's Bar Marmont.

"[Sam] told security to look out for Lindsay and block her from getting in," a source tells Us. "Lindsay, of course...booked a room at [the hotel] Chateau and fully intended to crash the party and make a scene."

Bouncers kept the 22-year-old from entering the party, another source tells Us.

"She was heartbroken, but Sam did not want Lindsay entering and causing a scene and ruining Charlotte's night," the source says.

A source close to Lohan, though, insists it was the Ronson family who stayed in a room below her at the Chateau Marmont and "blasted[ed] music and would not let Samantha leave the room to meet Lindsay."